The present invention relates to a motor driving apparatus. More specifically, the present invention relates to an apparatus that employs a pulse width modulation (PWM) signal to drive a motor.
In a reading or writing apparatus for optical disks such as a compact disk-read only memory (CD-ROM), a CD-rewritable (CD-RW), a digital video disk-ROM (DVD-ROM), a DVD-random access memory (DVD-RAM), a DVD-RW and a mini disk (MD), a plurality of motors are typically implemented. Examples include a DC motor or a spindle motor for rotating an optical disk, a sled motor for moving an optical pickup, a loading motor for loading an optical disk and actuators such as tracking and focus for reading or writing signals.
FIG. 1 shows a conventional motor driving apparatus. As shown in FIG. 1, BTL (Balanced Transformerless) drive technology is usually employed in the conventional motor driving apparatus.
An analog signal is applied to an input terminal 5 of the BTL driving circuit, and is compared with a reference voltage (Vref) to determine levels and also polarities of a voltage and a current. A digital controller (not illustrated in FIG. 1), for example, a digital signal processor or a micro computer, is usually used to perform calculations of digital signals to determine the input of the BTL drive type circuit. Consequently, a digital to analog (D/A) converter or a digital controller equipped with a D/A converter is required, thereby increasing the overall cost of the control system.
There is another problem in relation to the performance of the digital controller in the conventional apparatus. That is, the output voltage (Vout) is generally limited by three factors: 1) the power supply voltage (not illustrated in FIG. 1) of the BTL type drive circuit, 2) the input voltage of amplifiers 2 and 3, and 3) the gains of the amplifiers 2 and 3. The input voltage is generally lower than the power supply voltage of the digital controller because it is also an output voltage of the digital controller, and the gain of the amplifier is generally above 1.
Variations in the output voltage resulting from changes in the above-mentioned factors will be described below in detail.
For example, it is possible to obtain an input voltage of up to 5V in the case where the power supply voltage of the BTL type drive circuit is 12V, the saturation voltage of the amplifier is 2V, the gain of the amplifier is 2, and the maximum voltage, which can be applied to the motor, is about 10V.
First, the power supply voltage of the BTL drive circuit can limit the output voltage. When the power supply voltage of the BTL type drive circuit decreases to 5V, an input voltage above 2.5V does not change the output voltage that is applied to the motor. This problem can be solved by adding a resistor in series to the input terminal 5 (in other words, by decreasing the gain of the amplifier).
Second, the power supply voltage of the digital controller can limit the output voltage. When the digital controller employs 3.3V as the power supply voltage and the gain of the amplifier is 2, the maximum voltage applied to the input terminal 5 is 3.3V, and consequently the maximum output voltage is 6.6V.
As described above, with the conventional apparatus, the gain of the amplifier must be modified to avoid limitations in the output voltage, when the power supply voltage or the input voltage of the BTL drive circuit varies.
It is an object of the present invention to provide a motor drive apparatus, wherein a motor can be driven with a digital controller not equipped with a D/A converter; and a drive voltage applied to the motor can be controlled solely by a gain of an amplifier and a power supply voltage, with no dependence on the input signal from the digital controller.
To achieve the above object, the present invention provides a PWM input motor drive apparatus.
The PWM input motor drive apparatus in the present invention comprises motors, a digital controller, PWM decoders, duty capacitors, low pass filters and amplifiers.
The motor is used for converting electric energy into mechanical energy to carry out a predetermined function, and the digital controller generates PWM input signals with a predetermined duty factor.
The duty capacitors convert the PWM signals into driving voltages applied to the motors.
The PWM decoder generates a voltage across the duty capacitors by charging the duty capacitors during one predetermined state (either high or low) of the PWM signal and discharging the duty capacitors during the other state of the PWM signal.
The low pass filter removes an AC component of the voltage across the duty capacitors, and consequently converts the voltage into a DC drive signal.
The preferred PWM decoder in the present invention comprises a signal transformation device, a first switch, a second switch, a first current source, a second current source and a signal adjustment resistor.